A method of producing copies using an electrophotographic process has been disclosed in the prior art. Typically, in the electrophotographic process, an insulating, photoconducting surface is charged and after image-wise exposure, the image is developed with a toner and then transferred to a piece of paper, producing the finished copy.
An example of a prior art electrophotographic copying process is described in United Kingdom Pat. No. 940,577. This patent utilizes a photoconductive layer applied to a conductive support. The photoconductive layer has a pattern of areas. After charging, these areas of the photoconductive layer discharge more rapidly than the surrounding areas resulting in a pattern of small charged areas. Image-wise exposure of the photoconductive layer discharges some of the small charged areas forming a charge image in which the image portions are divided up into a large number of small charged areas.
The edges of the charge image are developed equally by an electrically conductive developing power. If the pattern of areas has been chosen to be sufficiently fine, a homogeneously developed powder image will be formed. Without the pattern of more rapidly discharging areas, the edges of the powder image would be developed more than the centers. This difference in the amount of developing would be visible on the copy in the form of a darker edge around the lighter image, particularly in the larger portions of the image.
Another prior art electrophotographic process is described in European Pat. No. 18,742. This patent also forms a pattern of charge areas in a charge image. The pattern of charge areas is produced by exposing a charged photoconductive element to homogeneous light passed through a screen, or by including a screen in the photoconductive element. The maximum density of the charge image portion is increased upon development with a conductive developing powder because of the pattern of charge areas.
Other possible prior art includes IBM Technical Disclosure Bulletin, Vol. 18, No. 10, March 1976 pages 3164-3165 and Xerox Disclosure Journal, Vol. 5, No. 2, March-April 1980 page 131.
While the electrophotographic processes described in the above references generally produce suitable copies, they have the distinct disadvantage that charge image portions corresponding to the light-gray parts of the original are either developed unevenly or not at all. This uneven development occurs primarily in the processes in which the charged photoconductive element contains a pattern of conductive areas or is exposed to homogeneous light passed through an optical screen.
Further, those processes utilizing a screen in the form of electrically insulating areas located on or in a photoconductive layer have the disadvantage that they cannot produce copies with a completely white background except by a long exposure. Complete discharge of this kind of photoconductive layer becomes difficult the shorter the length of exposure because the photoconductive layer discharges upon exposure from the top downward.
It is, therefore, an object of the present invention to obviate these disadvantages by providing a copying process which reproduces homogeneously without an extra long exposure not only the black areas of an original but particularly the light-grey and white areas. This ability to reproduce homogeneously the light-grey and white areas of an original is particularly important for copying drawings which require as white a background as possible and which also contain thin lines that are reproduced as grey due to the loss of contrast caused by the optical system used in the copying process. Moreover, in the extreme case in which a copy is required of a very low-contrast pencil drawing, which cannot be copied on a white background, it is possible with the present invention to produce a copy with an acceptable light-grey background because the background is uniformly reproduced.